Well, luckily I had lots of room, so I set one of those 12" X 12" patio pavers in the brooder and put my water on it. The chicks had to jump up on the paver to get to the water. While I still occasionally got a little bit of wood chips in the water, it was little enough that I could just scoop it out with my finger. I still changed the water every day to make sure it was fresh. A couple of small pieces of 2 X 4 set side to side would accomplish the same thing.
I am glad to hear your chicks are spreading out and active. At first I was worried your brooder was so small that you were heating the whole thing up to the max. Heating just one corner so that the chicks can go to it when they want is a method many people use. I had enough room to give them almost the entire coop with a heat lamp in just one corner. It will be harder when I try to bring in my next batch as I will have to confine them a little more. Do your chicks EVER go directly under the lamp to stay for awhile? I used the same kind of lamp you are, but I know a lot of people use much smaller bulbs. If you do a search you can probably find several threads where they discuss it.
I would think that at 3 weeks you could probably eliminate the heat lamp. Just my opinion. At that age I only used the heat lamp at night and that was in March in Colorado, in temps varying from the 20's to the 50's. The birds were fine.